The former commander of U.S. forces in Iraq and Afghanistan has been quietly urging U.S. officials to consider using so-called moderate members of al Qaeda’s Nusra Front to fight ISIS in Syria, four sources familiar with the conversations, including one person who spoke to Petraeus directly, told The Daily Beast.

The heart of the idea stems from Petraeus’s experience in Iraq in 2007, when as part of a broader strategy to defeat an Islamist insurgency the U.S. persuaded Sunni militias to stop fighting with al Qaeda and to work with the American military.

The tactic worked, at least temporarily. But al Qaeda in Iraq was later reborn as ISIS, and has become the sworn enemy of its parent organization. Now, Petraeus is returning to his old play, advocating a strategy of co-opting rank-and-file members of al Nusra, particularly those who don’t necessarily share all of core al Qaeda’s Islamist philosophy.

However, Petraeus’s play, if executed, could be enormously controversial. The American war on terror began with an al Qaeda attack on 9/11, of course. The idea that the U.S. would, 14 years later, work with elements of al Qaeda’s Syrian branch was an irony too tough to stomach for most U.S. officials interviewed by The Daily Beast. They found Petraeus’s notion politically toxic, near-impossible to execute, and strategically risky.

It would also face enormous legal and security obstacles. In 2012, the Obama administration designated al Nusra a foreign terrorist organization. And last year, the president ordered airstrikes on al Nusra positions housing members of the Khorasan Group, an al Qaeda cadre that was trying to recruit jihadists with Western passports to smuggle bombs onto civilian airliners.

Yet Petraeus and his plan cannot be written off. He still wields considerable influence with current officials, U.S. lawmakers, and foreign leaders. The fact that he feels comfortable recruiting defectors from an organization that has declared war on the United States underscores the tenuous nature of the Obama administration’s strategy to fight ISIS, which numerous observers have said is floundering in search of a viable ground force.

According to those familiar with Petraeus’s thinking, he advocates trying to cleave off less extreme al Nusra fighters, who are battling ISIS in Syria, but who joined with al Nusra because of their shared goal of overthrowing Syrian President Bashar al Assad.

Petraeus was the CIA director in early 2011 when the Syrian civil war erupted. At the time, he along with then-Secretary of State Hillary Clinton and Defense Secretary Leon Panetta reportedly urged the Obama administration to work with moderate opposition forces. The U.S. didn’t, and many of those groups have since steered toward jihadist groups like the Nusra Front, which are better equipped and have had more success on the battlefield.

How precisely the U.S. would separate moderate fighters from core members and leaders of al Nusra is unclear, and Petraeus has yet to fully detail any recommendations he might have.

Petraeus declined a request to comment on his views from The Daily Beast.

“This is an acknowledgment that the U.S. stated goal to degrade and destroy ISIS is not working. If it were, we would not be talking to these not quite foreign terrorist groups,” Christopher Harmer, a senior naval analyst with the Middle East Security Project at the Washington, D.C.-based Institute for the Study of War, told The Daily Beast. “Strategically, it is desperate.”

Privately, U.S. officials told The Daily Beast that any direct links with al Nusra are off the table. But working with other factions, while difficult, might not be impossible.

Still, the very forces that Petraeus envisions enlisting, and who may have once been deemed potential allies when they were fighting Assad, now may be too far gone. Moreover, there is no sign, thus far, of a group on the ground capable of countering ISIS, at least without U.S. assistance.

“As prospects for Assad dim, opposition groups not already aligned with the U.S. or our partners will face a choice,” one U.S. intelligence official told The Daily Beast. “Groups that try to cater to both hard-liners and the West could find themselves without any friends, having distanced themselves from groups like al Qaeda but still viewed as extremists by the moderate opposition and their supporters.”

News of Petraeus’s proposal comes at a potentially opportune moment for the Obama administration as it looks toward some resolution of the civil war in Syria. On Friday, Ambassador Michael Ratney, the newly-minted U.S. special envoy to Syria, set out to meet with Russian, Saudi, and United Nations officials in search of a political settlement to the conflict.

Like Petraeus, Ratney is in search of partners. He’s “trying to come up with options for some sort of political process, a political process that we know is going to have to include opposition groups and try to work through what that means and what that’s going to look like,” State Department spokesman John Kirby told reporters last week. Kirby stopped short of saying just which opposition groups should be part of the discussion.

The U.S. has insisted that any negotiated settlement must not include Assad, even as Russia has hinted Assad must be a part of a deal. Assad himself said in a television interview last week that he will not work with U.S. allies in Turkey and Saudi Arabia.

On the ground, the two most powerful anti-Assad forces are ISIS and al Nusra, and the U.S. won’t negotiate with either.

Petraeus’s strategy depends on a number of key assumptions, chiefly that U.S. intelligence and military officials would be able to distinguish who among al Nusra’s ranks is truly moderate and doesn’t share the terrorist group’s goal of replacing Assad with an Islamist government.

The former general isn’t the only ex-official who wants to talk to jihadist-linked fighters who share some, if not all, of the United States’ goals.

Robert Ford, the former U.S. ambassador to Syria, has called for dialogue with Ahrar al Sham, a jihadist force he has called “probably the most important group fighting the Syrian regime now.”

In a recent article for the Middle East Institute, Ford said that the capture of the Syrian provincial capital of Idlib last March, which was attributed by some to al Nusra, really should be credited to Ahrar, which had more fighters in the battle.

“Ahrar is a key force on the battlefield, but Western media allots little space to describe it beyond saying it is hard-line or jihadi,’” Ford wrote. That label, he acknowledged, stems from Ahrar calling for an Islamic state in Syria, as well as its collaboration with al Nusra against Assad and ISIS. The group was also founded by a former deputy to the current al Qaeda leader, Ayman al-Zawahiri.

But, Ford insisted, “Ahrar is not a junior partner of Nusra; there are ideological and political differences between them.”

Some U.S. intelligence officials disputed that, and said Ahrar is currently on a charm offensive, trying to distance itself from Islamic groups like al Nusra and thus win support in Washington while it looks forward to grabbing power after Assad falls.

“Some groups will look to pave their way to a seat at the post-Assad table by seeking public support, such as Ahrar al Sham, while others will affirm their choice through their actions,” the U.S. intelligence official said.

The extent to which the U.S. opposes working with Ahrar, a group that swears it’s independent, points out just how difficult it would be to recruit members of al Nusra, which is al Qaeda’s official affiliate in Syria.

And yet that’s not out of the question. The more extreme ISIS becomes, the more other hard-line groups seem to soften by comparison. ISIS, with its filmed executions, organized kidnappings, and enslavement of women and girls, has become so barbaric that it has been isolated from other fighting groups on the ground, said Harmer, the military analyst.

“Alliances of convenience that would have been impossible two years are now plausible, and in some ways inevitable, because we are not willing to put boots on the ground,” Harmer said.

Al Nusra has played an arguably helpful role to the U.S. already, albeit indirectly and behind the scenes. In 2014, officials in Qatar reached out to their contacts with al Nusra to help free American journalist Peter Theo Curtis, multiple sources, including former U.S. officials familiar with the negotiations, have told The Daily Beast. Al Nusra elements were operating so closely with the American-backed Free Syrian Army at that time that American warplanes almost hit the moderate rebels as it was targeting the jihadists.

The U.S. has tried other means to field a sustainable ground force to confront ISIS. So far, none of them have worked reliably. The most successful ground force so far has been the YPG, a Kurdish element, which drove ISIS out of the northern Syrian city of Kobani and other nearby cities under the cover of U.S. airstrikes.

But since the U.S. struck a deal to allow combat flights from Turkey, which opposes emboldening Kurdish forces, doubts have surfaced over whether the U.S. would keep providing air support for the YPG as its seeks to take Syrian territory. So far, the YPG has not pushed for any more land, instead defending what it already has.

U.S. efforts to train local forces in Syria have faltered, as well. The first batch of 54 fighters trained by American military forces dissolved in August. Some fighters fled back to their homes in Syria. Others were captured by al Nusra. While the U.S. military has said it’s still training fighters, privately officials concede the group has fallen far short of expectations. At one point, the U.S. planned to train 15,000 fighters in three years.

“We should under no circumstances try to use or co-opt Nusra, an Al Qaeda affiliate in Syria, as an organization against ISIL,” Petraeus said. “But some individual fighters, and perhaps some elements, within Nusra today have undoubtedly joined for opportunistic rather than ideological reasons: they saw Nusra as a strong horse, and they haven’t seen a credible alternative, as the moderate opposition has yet to be adequately resourced.”

Petraeus said the U.S. should try “splintering [Al Nusra’s] ranks by offering a credible alternative to those ‘reconcilable’ elements of those organizations.”